Important technical properties of an antenna structure are physical size and radiation efficiency. For example, an antenna of a cellular mobile phone is nowadays usually located inside the cover of the phone device. Especially in folding mobile phones, e.g. in a clamshell type phone, the thickness of the antenna structure is an important quantity. This is due to the fact that a phone device should be thin enough also in a folded state. Another important issue is the radiation efficiency. The radiation efficiency means the ratio of the power supplied to an antenna to the power radiated by the antenna. Small radiation efficiency means increased power consumption when a desired level of radiated power is generated. The power consumption is a crucial issue especially in battery-energized devices like cellular mobile phones. In today's mobile phones an antenna may have to operate at several frequency bands. The frequency bands may be for example: 900 MHz GSM band, 1800 MHz band DCS (Digital Communication Service), and 1900 MHz PCS (Personal Communication Service) band. The radiated efficiency has to be good enough over all the frequency bands at which an antenna operates. Furthermore, it is advantageous if the radiating efficiency of an antenna at a desired frequency band is insensitive to existence of electrically conductive materials in the vicinity. For example in a folding phone application electromagnetic properties of the near-surroundings of an antenna depend in some extent on opening position of a phone mechanics.